There seems to be no way to select previous equations with the keyboard
After pressing enter, there seems to be no way to bring the equation back into the edit field, to tweak it, via keyboard. The only way is to click on its entry in the backlog.
A natural way to fix this would be to make pressing up restore the previous expression, as it does in terminals. Currently, pressing up in the edit field just makes an error sound.
Another fix would be to provide an option to simply not do the traditional calculator thing where the result of the expression replaces the expression in the equation field, instead, show the result of the expression in realtime, in the right column. (Detail: Don't show syntax errors in realtime. When there's a syntax error, show a blank field. Only show the syntax errors when the user presses enter)